I have been running SFOS on my Nexus 5 for about a week now. I can say that it is very usable. There really is only one huge issue and that is the camera is not working. Otherwise everything is working pretty well! I only have a couple of annoyances:

1. 4G/LTE data is not working (but voice is and 3G data works just fine)
2. BT connection issues
3. GPS is supposedly working but I have found no NAV/MAP solution for SFOS on a Nexus 5

The second one YMMV. I have issues with it staying connected to my handsfree system in my car (the connection will drop sporadically). The only time I have booted into Android is to either use MAPS or the Camera.

I kind of look at this forum as a place to discuss SFOS and not necessarily the Jolla Phone. Some of us don't have the ability to purchase the Jolla Phone due to market restrictions. This thread has been informative to me and maybe it can still be to someone else. What is the harm in leaving it unlocked?